1. Field of the Invention
The present invention relates to an image display apparatus, and more particularly to a spacer for use in the image display apparatus having an electron-emitting device.
2. Description of Related Art
Because a plane type image display apparatus having a thin depth among image display apparatus using electron-emitting devices is a space-saving image display apparatus and light in weight, the plane type image display apparatus is noticed as a substitute for a Braun tube type display apparatus. FIG. 9 is a perspective view showing an example of a display panel unit of a plane type image display apparatus. FIG. 9 shows the display panel unit with a panel being partially broken for the illustration of an inner structure.
In the drawing, a reference numeral 3115 denotes a rear plate. A reference numeral 3116 denotes a side wall. A reference numeral 3117 denotes a face plate. The rear plate 3115, the side wall 3116 and the face plate 3117 constitute an envelope (airtight container) for keeping the inside of the display panel to be a vacuum.
A substrate 3111 is fixed to the rear plate 3115. An N×M cold-cathode elements 3112 are formed on the substrate 3111 (N and M are severally a positive integer being two or more, and are suitably set according to the aimed number of display pixels). Moreover, the N×M cold cathode elements 3112 are wired by means of M pieces of row direction wiring 3113 and N pieces of column direction wiring 3114, as shown in FIG. 9. A portion composed of the substrate 3111, the cold cathode elements 3112, the row direction wiring (upper wiring) 3113 and the column direction wiring (lower wiring) 3114 are totally called as a multi-electron beam source. Moreover, insulation layers (not shown) are formed between both of the row direction wiring 3111 and the column direction wiring 3114 at least at positions where they intersect with each other, and electrical insulation between them is held.
A phosphor film 3118 made of phosphor is formed on the under surface of the face plate 3117. The three primary colors, red (R), green (G) and blue (B), of phosphor (not shown) are separately coated as the phosphor film 3118. Moreover, a black body (not shown) is formed between each of the colors of phosphor constituting the phosphor film 3118. Furthermore, metal-backing 3119 made of aluminum or the like is formed on the surface of the phosphor film 3118 on the side of the rear plate 3115.
Reference marks Dx1-Dxm, Dy1-Dyn and Hv denote electrical connection terminals of the airtight structure provided for connecting the display panel with a not shown electric circuit electrically. The terminals Dx1-Dxm, Dy1-Dyn and Hv are electrically connected with the row direction wiring 3113 and the column direction wiring 3114 of the multi-electron beam source, and the metal backing 3119, respectively.
Moreover, the inside of the airtight container is held to be a vacuum at the degree of about 1.3×10−4 Pa. As the display area of the image display apparatus becomes larger, means for preventing the rear plate 3115 and the face plate 3117 from being deformed or damaged owing to the difference of atmospheric pressure between the inside of the airtight container and the outside thereof becomes more necessary. A method of thickening the rear plate 3115 and the face plate 3116 generates distortion of an image and a parallax when the image display apparatus is seen from an oblique direction as well as the increase of the weight of the image display apparatus. Conversely, the image display apparatus shown in FIG. 9 is made of a relatively thin glass plate, and is equipped with structural supporting members (called as spacers or ribs) 3120 for supporting the glass plate against the air pressure. Thus, the distance between the substrate 3111, on which the multi-electron beam source is formed, and the face plate 3117, on which the phosphor film 3118 is formed, is ordinarily kept to be within a range of a submillimeter order to several millimeter order. Also, as described above, the inside of the airtight container is held to be a high vacuum.
When a voltage is applied to each of the cold cathode elements 3112 through the terminals Dx1-Dxm and Dy1-Dyn on the outside of the container of the above-described image display apparatus using the display panel, electrons are emitted from each of the cold cathode elements 3112. At the same time, a voltage within a range from several hundred volt to several kilovolt is applied to the metal backing 3119 through the terminal Hv on the outside of the container to accelerate the emitted electrons. The accelerated electrons collide with the inner surface of the face plate 3117. Thereby, each color phosphor constituting the phosphor film 3118 is excited to emit light, and an image is displayed.
However, there has been a problem that an image is displayed with distortion at positions near to the spacers. A part of the electrons emitted from the positions near to the spacers 3120 collides with the spacers 3120, or ions generated by the ionization caused by the operation of the emitted electrons attach the spacers 3120. By such causes, there is the possibility of causing spacer charging. The orbit of an electron emitted from a cold cathode element 3112 is bent by the charging of a spacer 1320, and the electron arrives at a position of the phosphor different from a correct position. Consequently, the image at the position near to the spacer is displayed with distortion. A method for solving the problem is disclosed in U.S. Pat. No. 5,760,538. In the method, a resistance film is formed on the surface of each spacer, and a small current flows through the resistance film for removing the charging of the spacer. Although the details of the causes of the charging have not been found, reflection electrons emitted from an electron-emitting device near to the spacer, and secondary electrons emitted from the surface of the spacer have been considered to be the causes. A method for improving the emission of these electrons is proposed in Japanese Patent Application Laid-Open No. 2000-311632.